This invention relates to systems and methods of accurately measuring low values of Polarization Mode Dispersion (PMD) in an optical fiber. Specifically, the method provides accurate optical fiber measurement of PMD in a low mode coupling condition.
It is well known that the so-called xe2x80x9csingle mode fiberxe2x80x9d that is commonly used in communication systems is not purely single mode. Rather, two modes, with perpendicular polarizations, exist in a single mode fiber. See, for example, Dandliker, R., Anisotropic and Nonlinear Optical Waveguides, C. G. Someda and G. Stegeman (editors), Elsevier, N.Y., 39-76, 1992. Mathematically, these two polarizations form an orthogonal basis set. Accordingly, any configuration of light that propagates through a single mode fiber can be represented by a linear superposition of these two modes.
If the fiber is perfectly circularly symmetric in both geometry, and internal and applied stress, the two polarization modes are degenerate. They propagate with the same group velocity and have no time delay difference after traveling the same distance in the fiber. However, in practice, an optical fiber is not perfectly circularly symmetric. Imperfections such as geometric and form deformation and stress asymmetry break the degeneracy of the two modes. See, for example, Rashleigh, S. C., Journal of Lightwave Technology, LT-1:312-331, 1983. As a result, the two polarization modes propagate with different propagation constants xcex21 and xcex22. The difference between the propagation constants is termed birefringence (xcex94xcex2) and the magnitude of the birefringence is defined y the difference in the propagation constants of the two orthogonal modes:
xcex94xcex2=xcex21xe2x88x92xcex22
Birefringence causes the polarization state of light propagating in the fiber to evolve periodically along the length of the fiber. The distance required for the polarization to return to its original state is the fiber beat length (Lb), which is inversely proportional to the fiber birefringence. In particular, the beat length Lb is given by:
Lb=2xcfx80/xcex94xcex2
Accordingly, fibers with greater birefringence have shorter beat lengths and vice versa. Typical beat lengths observed in practice range from as short as 2-3 millimeters (a high birefringence fiber) to as long as 10-50 meters (a low birefringence fiber).
In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence means that the two-polarization modes travel at different group velocities, the difference group delay (DGD) increasing as the birefiingence increases. Randomness of the birefringence creates a statistical distribution of the DGDs. The statistical average of the DGDs between the two polarization modes is called polarization mode dispersion, or PMD. PMD causes signal distortion that is detrimental for accurate signal transmission for both high bit rate digital systems and analog communication systems. Various mechanisms have been identified for reducing PMD in optical cable during manufacturing, such as spinning the optical fiber during manufacturing. See, for example, Systems and Methods for Forming Ultra-Low PMD Optical Fibers Using Amplitude and Frequency Keyed Fiber Spin Functions, application Ser. No. 10/202,540, filed on Jul. 23, 2002 by the same assignee which is incorporated by reference into this application. It is therefore desirable to measure the PMD levels of an optical fiber cable.
The intrinsic birefringent characteristic of a fiber can be impacted by a variety of factors, including not only the above mentioned spin during draw, but fiber core ovality, non-axisymmetric mechanical stress caused by factors internal to the fiber, and extrinsic effects (bending, twist, tension force, temperature variation etc.). (See S. C. Rashleigh, xe2x80x9cOrigins and Control of Polarisation Effects in Single-Mode Fibers,xe2x80x9d J. Lightwave Tech., LT-1, No. 2, (1983) p. 312-331.) Though both intrinsic and extrinsic effects play a role in any practical fiber deployment, it is desirable to measure the intrinsic PMD of an optical fiber. This allows a manufacturer to publish a fiber""s PMD specification independent of any particular cable geometry and provides a metric by which process improvement may be measured in manufacturing the optical fiber.
It has long been known that PMD measurements of useful fiber lengths on shipping spools (which are typically 160 mm in diameter) do not reflect the intrinsic fiber PMD. These measurements are strongly-influenced by external effects on the fiber, such as tension, (from a normal force of 30-40 grams from spooling) and mode coupling that occurs because of fiber crossovers, the mode coupling is influenced by twist on the cable associated with spooling of the fiber. When the fiber is unspooled, these external effects are altered and an ideal relaxed fiber would be in a xe2x80x9clow mode coupledxe2x80x9d (LMC) condition. The best estimate of xe2x80x9cintrinsicxe2x80x9d PMD can be obtained by measurements under LMC condition. Hence, various so-called LMC techniques have been developed to measure PMD in alternative configurations. Three configurations have largely been developed for measuring PMD: xe2x80x9cquiescentxe2x80x9d cable, large diameter spool, and loose fiber. However, as it will be seen, each configuration presents some limitations.
The xe2x80x9cquiescentxe2x80x9d cable configuration measures optical fiber that has been incorporated into a cable, which comprises one or more optical fibers and various layers of sheathing. Cables are not generally considered to be valid LMC configurations because of the invariable external mechanical stresses imposed by the sheathing and application of colors during manufacturing. However, xe2x80x98loose tubexe2x80x99 cable configurations, while not entirely eliminating fiber twists and crossovers, do provide a lower mode-coupling environment for the fiber. Thus, fiber in cables of this sort, such as those used in undersea cables, should perform in a similar way as compared to the same fiber under true LMC conditions. But, many other cable configurations do not perform in a similar way as the same fiber under true LMC conditions. Further, the physical facilities for measuring long distances of cables may require large (30 ft diameter) pans for winding the cable in a LMC condition, and a conventional fiber manufacturer may not have these types of facilities.
The second configuration incorporates the use of large diameter spools (generally 300 mm or larger) on which the fiber is wound at low tension for providing a conventional LMC reading. However, bending birefringence is not entirely eliminated and zero tension on the fiber is impossible to obtain using a conventional rewinder apparatus. Further, the distance of fiber that can be measured is limited since only a single layer of fiber can be wrapped on the spool. Of course, larger and larger diameter spools can be used, but larger sizes complicate handling the spools and increases space requirements. Thus, depending on the spool surface area and size, the length of fiber that may be measured can be severely limited.
The third configuration for measuring fiber in a LMC condition is to measure the fiber in an unrestrained configuration by using loose coils or collapsible spools. This technique arranges the loose fiber on a large, flat surface where the fiber is spread out to allow zero tension and large bend radii. It is necessary that the fiber be in a xe2x80x98relaxedxe2x80x99 state to eliminate bending and tensioning stress as much as possible of the optical fiber. Frequently, a period of time is required for the fiber to xe2x80x98relaxxe2x80x99 in order to obtain accurate measurements. However, the physical arrangement of fiber in a xe2x80x98loosexe2x80x99 configuration is a potentially awkward arrangement and the length of fiber that can be measured is based on the size of the facility. In all three techniques, measuring short lengths fiber of a few kilometers is possible, but measuring larger lengths becomes increasingly logistically difficult.
Further complicating the measurement of PMD is that current optical fibers typically represents much lower PMD values than fibers manufactured a few years ago. Whereas fiber in the mid 1990""s exhibited PMD values typically greater than 0.5 ps/kmxc2xd, today""s fiber typically exhibits values in the 0.01-0.04 ps/kmxc2xd range. Consequently, many of the conventional measuring techniques and practices cannot accurately measure such low PMD values.
Standards bodies comprising industry experts, which proscribe uniform methods for measuring PMD, have advocated both use of the large diameter spool and the loose coil method where the fiber is placed in a racetrack configuration. These recommendations have largely ignored the practical aspects of handling fiber in such an arrangement and/or have relied upon current commercial test equipment that requires a high minimum measurable PMD.
Consequently, there is a need to easily measure PMD in optical fiber in a rapid and logistically simplified manner yielding accurate measurement results without being constrained to relatively short lengths of optical fibers.
A method is disclosed for measuring the polarization mode dispersion of an optical fiber using the steps of introducing a plurality of localized external perturbations (LEP), measuring the differential group delay, altering the LEP, measuring the differential group delay, and repeating the steps sufficiently to produce a polarization mode dispersion value.
Further, an example system is disclosed for measuring the polarization mode dispersion of an optical fiber comprising of a device for introducing LEP onto a fiber, a device for measuring the differential group delay, and a processor receiving the measurement and processor calculating a polarization mode dispersion value.